The present invention relates to circuits and methods for rapidly recharging batteries in portable computers.
Many battery technologies, such as nickel/metal hydride ("NiMH"), achieve their best lifetime and energy storage if they are not recharged at too high a rate. For ordinary NiMH batteries, it is preferable not to recharge them any faster than their "1 C" rate. (This is the rate which will theoretically charge a battery to its capacity in one hour; thus, for a 1.7 Ampere-hour battery the 1 C rate is 1.7 Amperes.) The characteristics of NiMH batteries, and of many other battery types, are described in Linden, HANDBOOK OF BATTERIES (2.ed. 1995), which is hereby incorporated by reference.
Modern portable computers typically include an AC adapter which, when the computer is plugged into a mains power socket, derives DC power from the AC input. This DC power is used to power the electronics, and also to recharge the battery or batteries in the computer. As modern portable computers are loaded with more powerful electronics and more accessories, their AC adapters have become correspondingly more powerful. A surprising side-effect of this is that the AC adapters are sometimes powerful enough to provide too much current to the batteries when the computer is off.
System with Limited Long-Time-Averaged Battery Charging Rate
The present application describes a battery charging system in which the battery charge rate is limited in a very simple way. The present inventors have realized that the maximum charging rate can be averaged over a very long time window. Accordingly, the present application teaches that the battery charging rate can be limited to an appropriate level merely by: measuring the charging current over a fixed long period (10 seconds, in the presently preferred embodiment), and when the maximum charge for one period has been transferred, then cutting off current for the remainder of the fixed long period.
This method permits good control of battery charging to be performed with minimal load on the controller logic. Thus this control algorithm can easily be performed by a microcontroller or microprocessor which has many other supervisory functions.
The present inventors have obtained confirmation that this technique produces an internal pressure profile (within the battery) which is very similar to that of continuous charging at the same average rate. Thus battery degradation is easily avoided.
Of course, the target charging rate does not have to be a 1 C rate. At least one author has suggested that charging rates up to 5 C can be used for NiMH batteries. (This is a very aggressive specification, and would not be used in most systems.) However, whatever the target charging rate, the present application provides a new way to achieve it.
This innovative charging step does not have to be the only charging step, and can optionally be preceded or followed by charging steps which use completely different control algorithms. Many such control algorithms are detailed in the HANDBOOK OF BATTERIES cited above.